I was a bit sketchy with mounting my m2-atx on it's side using normal standoff relying on the thread to kepp it in, so I used
these which screw in from either side so it's not going anywhere. The ones in the link are 10mm long, but the ones I used are only 5mm long and came in a pack of 10.
If you were going to mount anything to the side of the box of upside down, these would work well.